Imaging probe and method of obtaining position and/or orientation information

ABSTRACT

A method of obtaining information about the position and/or orientation of a magnetic component relatively to a magnetometric detector, the magnetic component and the magnetometric detector being moveable independently from each other relatively to a static secondary magnetic field, the method comprising the steps of: measuring in the presence of the combination of both the magnetic field of the magnetic component and the static secondary magnetic field essentially simultaneously the strength and/or orientation of a magnetic field at at least a first position and a second position spatially associated with the magnetometric detector, the second position being distanced from the first position; and combining the results of the measurements to computationally eliminate the effect of the secondary magnetic field and derive the information about the position and/or orientation of the magnetic component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/342,748, filed on May 28, 2014, now pending, which is a U.S. NationalStage Application of PCT/EP2011/065420, filed Sep. 6, 2011, thedisclosures of which are both incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The invention relates to methods of obtaining information about theposition and/or orientation of a magnetic component relatively to amagnetic detector. It further relates to systems of an imaging probe forimaging at least part of the tissue of a patient and a magnetic detectorfor detecting the position and/or orientation of the magnetic componentrelatively to the magnetometric detector. It moreover relates to amedical device at least a portion of which is insertable into the tissueof the patient, the medical device comprising a magnetic component, andto a method of obtaining position and/or orientation information aboutat least a part of a medical device. Finally, the invention relates toan apparatus for magnetizing an elongate medical device.

BACKGROUND OF THE INVENTION

In numerous medical procedures that involve the insertion of a medicaldevice into a patient's tissue, e.g. minimally invasive procedures andlocal anesthesia, it can be of great advantage for the physician to beinformed of the exact position of the medical device in the patient'stissue. For example, to introduce regional anesthesia, includingperipheral nerve blocks for surgical anesthesia or postoperativeanalgesia, a needle can be guided to the region of interest with thehelp of ultrasound imaging. It has proven challenging, however, toprecisely detect the needle's end point in the ultrasound image.

Northern Digital Inc., Ontario, Canada (www.ndigital.com) offers anelectromagnetic detection system under the trade name “Aurora”. Thesystem comprises a field generator for creating an electromagnetic fieldand various types of sensor coils that react to the field produced bythe generator. One or more of the sensor coils can be embedded into amedical instrument such as a biopsy needle, a catheter or a flexibleendoscope for measuring in real time the position of the instrument'stip or, if several coils are embedded, the shape of the instrument. Thevarious types of sensor coils available differ in shape and size and candetect their position relatively to the generator's electromagneticfield in three-dimensional space and their orientation in two or threedimensions. Wires connect the sensor coils with a sensor interface unitthat transmits the coils' data to a system control unit. The systemcontrol unit collects the information obtained from the sensor coils andcalculates their position and orientation.

In “Evaluation of a miniature electromagnetic position tracker”, Mat.Phys. (2002), 29 (1), 2205 ff., Hummel et al. have studied the effectsof the presence of an ultrasound scan head on the accuracy of the“Aurora” electromagnetic tracking system measurement results.

Placidi, G. et al. in “Review of Patents about Magnetic LocalizationSystems for in vivo Catheterizations”, Rec. Pat. Biomed. Eng. (2009), 2,58 ff., distinguish between systems where the magnetic field is locatedoutside the patient's body (“extra-body generated magnetic field” as inthe “Aurora” system) and systems where the magnetic field is generatedby a permanent magnet located inside the patient's body (“intra-bodypermanent magnet”). A system is discussed that can detect the locationin three dimensions and the orientation in two dimensions of a permanentmagnet that is permanently fixed to an intra-body medical device. Eachmeasurement involves at least two spatially separated three-axismagnetic sensors in order to measure x-, y- and z-components of themagnetic field produced by the permanent magnet in at least two spatialpositions. Six magnetic sensors are arranged in a circle surrounding thepatient in order to ensure that each part of the patient's body iscovered by at least two of the sensors. Before use, the system iscalibrated to take into account the terrestrial magnetic field. In thecalibration step, in the absence of the permanent magnet, theterrestrial magnetic field is measured and then subtracted from eachsubsequent measurement. From the remainder, the position of the magnetis calculated. It is considered a disadvantage of the system that itcannot be moved once calibrated.

Yet, the U.S. Pat. No. 6,263,230 B1, which is cited in Placidi et al.,supra, B1 describes a “continuous automatic recalibration” scheme withwhich a detector can be moved after the initial calibration, even thoughnot simultaneously with the magnet. The magnetic detector system isattached to a fluoroscopic head in a known spatial relationship todetect the position of a permanent magnet of an indwelling medicaldevice and the magnet's field is approximated as a dipole field. Inorder to compensate for the terrestrial magnetic field as well aslocalized perturbations associated with this field, an initialcalibration is performed before the magnet is introduced into thepatient. For each magnetic sensor of the detector system an offset valueis determined. Later, when the magnet has been introduced into thepatient, the offset values are subtracted from the readings of themagnetic sensors, thus compensating for the terrestrial magnetic fieldand its localized perturbations. Moreover, the “continuous automaticrecalibration” scheme allows compensating for the localizedperturbations of the terrestrial magnetic field even if the detectorsystem is moved: According to this scheme, the detector is moved whilethe magnet remains stationary at its position that is known from theprevious measurement. The exact positional change of the detector istracked by a digitizing arm and from this the magnetic field at thedetector's new location due to the magnet is calculated. The result issubtracted from the field actually measured by the detector and theremainder is considered the contribution of the terrestrial magneticfield at the new location. The process can be repeated as the detectoris moved to yet another location.

U.S. Pat. No. 6,216,029 B1 discloses an apparatus for ultrasoundfree-hand directing of a needle. Both an ultrasound probe and the needleor a needle guide are provided with orientation sensors for sensing theposition of the probe and the needle with respect to a reference. Theorientation sensors each may comprise three transponders in triangularalignment. The transponders preferably are electro-optical sensors whichoperate with infrared or visible light. Alternatively, the systemcomprises a magnetic transmitter and magnetic receivers attached to anultrasound probe and the needle or needle guide. On a displays screen,the ultrasound image of a target area is shown. Moreover, the needle isshown as a distinctly coloured line, even if the needle is outside theultrasound image. In addition or alternatively, a trajectory of theneedle is displayed.

Similarly, U.S. Pat. No. 6,733,468 B1 discloses a diagnostic medicalultrasound system in which both an ultrasound probe and an invasivemedical device, e.g. a cannula, have location sensors attached to themfor sensing their position and/or orientation. From the positions of theneedle and the probe the relative position of the needle with respect tothe probe's imaging plan is determined. From this, a projected and anactual trajectory of the invasive medical device are calculated andsuperimposed on the ultrasound image.

Problem to be Solved by the Invention

It is an objective of the present invention to provide improved methodsof obtaining information about the position and/or orientation of amagnetic component relatively to a magnetometric detector. The inventionfurther aims to provide improved systems of a imaging probe for imagingat least part of the tissue of the patient and a magnetometric detectorfor detecting the position and/or orientation of a magnetic componentrelatively to the magnetometric detector. The invention also seeks toprovide an improved medical device at least a portion of which isinsertable into the tissue of a patient, the medical device comprising amagnetic component, and to provide an improved method of obtainingposition and/or orientation information about at least a part of amedical device. Finally, it is an objective of the invention to providea new apparatus for magnetizing an elongate medical device.

Solution According to the Invention

In the following, the present invention is described with reference tothe claims. Note that the reference numbers in all claims have nolimiting effect but only serve the purpose of improving readability.

Methods of Obtaining Position and/or Orientation Information

According to one aspect of the invention, the problem is solved byproviding a method where the position and/or orientation of the magneticcomponent relatively to the magnetometric detector is obtained directly.Advantageously, because the effect of the secondary magnetic field iscomputationally eliminated by combining the results of the at least twosimultaneous measurements, the initial calibration step that is forexample used in the method described in U.S. Pat. No. 6,263,230 B1 forobtaining offset values to compensate for the terrestrial magnetic fieldis no longer required. Also, the “continues automatic recalibration”procedure disclosed therein, which relies on a digitizing arm to measurethe detector's positional change and moreover requires that the magneticcomponents remains stationary while the detector is moved, can beavoided. Rather, the position and/or orientation of the magneticcomponent can be derived even if the magnetometric detector and themagnetic component are moved simultaneously. This is of considerablebenefit, in particular when the magnetometric detector is attached to ahand held probe such as an ultrasound probe for ultrasound-assistedmedical procedures to track the position of a medical device relativelyto the image created by the probe of the tissue of the patient. In suchcases, it is almost impossible for the physician to keep the probestationary while the medical device is moved. Moreover, as thedigitizing arm of U.S. Pat. No. 6,263,230 B1 can be dispensed with,advantageously, the means for detecting the position and/or orientationof a magnetic component according the invention do not require physicalcontact with a reference. In fact, it is achievable that for providingthe desired information about the position and/or orientation of amagnetic component no means as a reference other than the magneticcomponent is required. This is in contrast not only to the teaching ofU.S. Pat. No. 6,263,230 B1 but also to that of e.g. U.S. Pat. No.6,216,029 B1 and U.S. Pat. No. 6,733,468 B1. As the quantity ofinterest, namely the position and/or orientation of the magneticcomponent relatively to the probe, is obtained directly, the estimationis less error-prone than estimation methods that rely on separateestimates for the probe position and the position of the magneticcomponent, e.g. the methods disclosed in U.S. Pat. No. 6,216,029 B1 andU.S. Pat. No. 6,733,468 B1.

Also advantageously, by combing the results of the two measurementstaken essentially simultaneously to obtain the position and/ororientation of the magnetic component, procedures can be eliminated thatrely on an oscillating magnetic field of the magnetic component, e.g.the methods disclosed in Placidi, G. et al., supra, in relation toextra-body generated magnetic fields, in order to compensate for theterrestrial magnetic field.

In the context of the present invention, a “magnetometric detector” is adevice that can obtain quantitative information about the magnetic fieldto which it is exposed, such as the absolute value, the direction and/orthe gradient of the magnetic field. The magnetometric detector maycontain one or more magnetometers. The expression “spatially associated”in relation to the positions at which the measurements take place andthe magnetometric detector means that the positions move in synchronywith the detector (and consequently with each other) so that from thelocation and orientation of the positions that of the detector can bederived.

The “secondary magnetic field” will in general comprise the terrestrialmagnetic field. In addition, it might comprise distortions in theterrestrial magnetic field or other magnetic field, e.g. created byapparatus in the vicinity of the magnetometric detector. The preferredsecondary magnetic field is essentially homogeneous within the space inwhich the magnetometric detector moves when used.

A “magnetic component” is an entity that creates its own magnetic field.Due to its magnetic property the magnetic component can provide themagnetometric detector with information about its position and/ororientation.

“Information about the position” of the magnetic component refers to theposition in at least one spatial dimension, more preferably in two, morepreferably in three dimensions. Similarly, the “information about theorientation” of the magnetic component refers to the orientation in atleast one spatial dimension, more preferably in two, more preferably inthree dimensions. The obtained information preferably is the positionand/or orientation of the magnetic component within a certainresolution. Yet, merely the information as to whether the magneticcomponent is in an imaging plane of the imaging probe or not wouldalready constitute position information within the scope of the presentinvention. Moreover, information of whether the magnetic component is infront of or behind the imaging plane constitutes position information.

The problem according to the invention is also solved by providing amethod where the measurement of the inertial measurement unit theorientation or even both the orientation and position of themagnetometric detector. From the result, the orientation or orientationand strength, respectively, of the secondary magnetic field, preferablythe terrestrial magnetic field, relatively to the detector can bederived. For this purpose, preferably, in an initial calibration stepthe orientation or strength and orientation of the secondary magneticfield relatively to the magnetometric detector are measured in theabsence of the magnetic component. By tracking the orientation changesof the magnetometric device from the initial calibration position onecan calculate the components of a secondary magnetic field which isapproximately stationary in space and time.

Within the context of the present invention, an “inertial measurementunit” is a unit that comprises a gyroscope and/or an accelerometer,preferably both. Advantageously, for providing the desired informationabout the position and/or orientation of a magnetic component no meansas a reference other than the magnetic component is required.

Systems of an Imaging Probe and a Magnetometric Detector

According to another aspect of the invention, the problem according tothe invention is moreover solved by providing a system including animaging probe for imaging at least part of the tissue of a patient and amagnetometric detector for detecting the position and/or orientation ofa magnetic component relative to the magnetometric detector,characterized in that the imaging probe is a hand held probe.

A “hand held probe” is a probe that the use is intended for the user tobe held in the desired position by hand. In particular, in a hand heldprobe, technical means such as a support arm, a runner or a wire arelacking that would hold the probe in position should the user remove hisor her hand. The preferred hand held probe comprises a handle.

Further, the problem according to the invention is solved by providing asystem with an imaging probe for imaging at least part of the tissue ofa patient and a magnetometric detector for detecting the position and/ororientation of a magnetic component relatively to the magnetometricdetector, characterized in that the system further comprises at leastone fastener for removably attaching the imaging probe to themagnetometric detector. Due to the fastener, the magnetometric detectoraccording to the invention can be attached to an imaging probe that wasoriginally not designed for the use (or at least not the exclusive use)with the magnetometric detector according to the invention. Preferably,the fastener is fixedly attached to the imaging probe and/or themagnetometric detector. Fasteners may also be provided as separate partsand may comprise a portion (e.g. self-adhesive of portion) for fixedlyattaching the fastener to the imaging probe and/or the medical device.

Medical Device and Method of Obtaining Position and/or OrientationInformation about the Medical Device

According to yet another aspect of the invention, the problem is alsosolved by means of providing a medical device with at least a portionwhich is insertable into the tissue of a patient, the medical devicecomprising a magnetic component that is integral with or removablyattachable to the tissue-insertable portion of the medical device,characterized in that the magnetic component is a functional componentof the medical device, and a method of obtaining position and/ororientation information about at least a part of a medical device bypermanently magnetizing a functional component of the medical device byexposing the functional component to a magnetic feld; and detectingposition and/or orientation information either of magnetic component bymeans of a magnetometric detector. The medical device and the method ofobtaining position and/or orientation information about at least a partof a medical device exploit the inventors' finding that many medicaldevices comprise functional components, e.g. a cannula or a metal rod,which can be magnetized in order to render the medical device detectableby a magnetometric detector. The functional component of the medicaldevice can therefore simply by means of magnetization be assigned to anadditional purpose beyond its original function in the medical device.

In the context of the present invention, a “functional component” of themedical device is a component that in addition to providing themagnetometric detector with position and/or orientation information alsocontributes to the functioning of the medical device, i.e. itcontributes to the medical device serving its purpose as a medicaldevice. In this regard, forwarding position and/or orientationinformation to the magnetometric detector is not considered a functionof the medical device. The function may e.g. be the transport of a fluidinto or out of the patient's tissue if the medical device is a catheteror a cannula, or the function may be an electrosurgical treatment if themedical device is an electrosurgical instrument.

The preferred magnetic component is an “essential” component of themedical device. In this context, “essential” means that the medicaldevice cannot fulfil its purpose when the magnetic component is removed.Alternatively, the magnetic component is not essential but neverthelessbeneficial. It may for example improve the functionality, the handlingor other properties of the medical device beyond providing orientationand/or position information to the magnetometric detector.

Magnetization Apparatus

Finally, according to a further aspect of the invention, the problem issolved by providing an apparatus for magnetizing an elongate medicaldevice with a reservoir for keeping the elongate medical device and amagnet for magnetizing the elongate medical device. The apparatus can beused to magnetize the elongate medical device, e.g. a cannula, justbefore a medical procedure is performed. Due to the apparatus comprisingboth a reservoir for keeping the elongate medical device and amagnetizer, cannula can easily be magnetized by e. g. a physician inorder to turn them into magnetic components for use in the methods, withthe system and in the medical device according to the invention.

While for better understanding of the fundamental concepts of theinvention throughout the present description and the claims reference ismade only to one magnetic component, the invention in all its aspects ofcourse also encompasses embodiments in which in addition furthermagnetic components are present. As is readily apparent to those skilledin the art, the methods, apparatus and systems according to theinvention can equally be applied to multiple magnetic components insteadof only one magnetic component.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Preferred features of the invention which may be applied alone or incombination are discussed in the following as well as in the dependentclaims.

Obtaining Position and/or Orientation Information

Preferably, the strength and/or orientation of the magnetic field asmeasured at one or more of the positions spatially associated with themagnetometric detector is used as a direct estimate of the strengthand/or orientation of the secondary magnetic field, i.e. the (possiblydistorted) terrestrial magnetic field. For this purpose, preferably,these position or positions are separated far enough away from themagnetic component to ensure that the measurement at this/theseposition(s) is sufficiently unaffected by the magnetic component'smagnetic field to directly provide an estimate of the strength and/ororientation of the secondary magnetic field. Therefore, if themagnetometric detector is integral with or attachable to a imagingprobe, e.g. an ultrasound imaging probe, as discussed further below, themeans (e.g. magnetometers) of the magnetometric detector for measuringthe strength and/or orientation of the secondary magnetic fieldaccording to the present embodiment of the invention is/are sufficientlyremote from the part of the imaging probe that is closest to the patientto ensure that the magnetometer(s) is/are essentially unaffected by themagnetic field of the magnetic component introduced into the patient'stissue.

In the context of this embodiment “direct estimate” means that themeasurements at the position or positions are sufficient to estimate,within the required accuracy, the strength and/or orientation of thesecondary magnetic field.

Advantageously, the magnetic field as measured according to the abovepreferred embodiment of the present invention can simply be subtractedfrom the result of the measurement at one or more other position(s) inorder to obtain for each of these other measurements the magnetic fieldof the magnetic component only. This can then be used to derive theposition and/or orientation of the magnetic component.

Preferably, the position and/or orientation of the magnetic component iscomputed by fitting a model of the magnetic field due to the magneticcomponent to the actual magnetic field of the magnetic component asobtained from the measurements at the positions affected by magneticcomponent's field after the secondary magnetic field has beensubtracted. Thus, position and/or orientation of the magnetic componentare the unknown parameters in the model fitting procedure.

In an alternative embodiment of the inventions, a model that comprisesthe secondary magnetic field, preferably a homogenous magnetic fieldrepresenting the terrestrial magnetic field, is a further unknownparameter in a model, and the model is fit by a suitable algorithm tothe results of the measurements at the positions in order to derive theunknown parameters, i.e. the position and/or orientation of the magneticcomponents and, if of interest, also the strength and/or orientation ofthe secondary magnetic field. This method is preferably employed if allpositions of the magnetometric detector at which the strength and/ororientation of the magnetic field is measured are considered to bepotentially affected by secondary magnetic field to a degree that noneof them can directly provide an estimate of the strength and/ororientation of the secondary magnetic field.

In a variation of the above method, the effect of the secondary magneticfield on the results of the measurements at the positions is firstcancelled out by determining differential values of the measurements atdifferent positions. For example, after normalization (as discussedbelow), the average of the strength and/or orientation of the magneticfield determined at the various positions can be subtracted from thestrength and/or orientation of the magnetic field at each individualposition. Hence, in effect the magnetometers function as gradiometers.These differential values can then be fit to a model that utilizesdifferentials or other functional derivatives of the original magneticfield.

Moreover, preferably, from the measurements, the change in orientationand/or position of the magnetometric detector due to a movement of themagnetometric detector is obtained. For example, the change inorientation and/or position of the magnetometric detector can beobtained from the orientation of the detector relatively to theterrestrial magnetic field as computed from combining the results of themeasurements at two or more positions of the magnetometric detector.Also, the orientation and position of the magnetometric detector can bederived from the measurements of an inertial measurement unit. If themagnetometric detector is attached to an imaging probe, this informationcan be used, for example, to combine the images acquired by the imagingprobe at different positions and/or in different orientations into athree-dimensional map or a panoramic map. In particular, this mayfacilitate three-dimensional mapping of extended volumes. Thus, theinertial measurement unit, in particular the accelerometer, on one handand the measurement of the strength and/or orientation of the magneticfield for estimating the secondary magnetic field on the other hand cansubstitute each other. However, the invention also comprises embodimentsin which both of these means are provided. In particular, the results ofboth means can be combined, e.g. averaged, to improve accuracy.

The Magnetometric Detector and the Base Unit

In the magnetometric detector the strength and/or orientation of themagnetic field preferably is measured, in at least two positions, morepreferably at least three, more preferably at least four positionsspatially associated with the magnetometric detector, the positionsbeing distanced from each other. The measurements may be combined toderive the position and/or orientation of the magnetic component. Theymay be moreover combined to computationally eliminate the effect of thesecondary magnetic field.

Preferably, the magnetic field in at least two positions, morepreferably at least three, more preferably at each of the positions ismeasured by a magnetometer of the magnetometric detector, eachmagnetometer being located at the respective position. Preferably, atthe first position, more preferably also at the second position, morepreferably also at the third position, more preferably at all positionsof the magnetometric detector the components of the magnetic field in atleast two linearly independent spatial directions, more preferably inall three linearly independent spatial directions are measured.

In a preferred embodiment of the invention, the results of themeasurements are transmitted to a base unit for processing, thepreferred base unit being separate from the magnetometric detector. Inthis context, “separate” means that the base unit and the magnetometricdetector do not move in synchrony with each other; in other words, theyare not spatially associated. Rather, the magnetometric detector canmove independently of the base unit. In particular, the base unit canremain stationary while the magnetometric detector (preferably attachedto the imaging probe as discussed above) is moved. The transmissionbetween the magnetometric detector and the base unit can be realised,for example, by a flexible cable or by a wireless connection. It is anadvantage of the wireless connection that the magnetometric detector canbe attached to a conventional imaging probe without requiring anothercable in addition to the probe cable.

It is an achievable advantage of this embodiment of the invention that alarge part or even all of the computation required to derive from theresults of the measurements the position and/or orientation of themagnetic component can be performed in the base unit. This is of benefitin view of the fact that the computational means required foreliminating the effect of the secondary magnetic field and deriving theposition and/or orientation of the magnetic component can be toodemanding for a microprocessor small enough to be easily attached to theimaging probe. Therefore, by shifting part or all of the computation tothe base unit where sufficient processing power can more easily beprovided, the magnetometric detector can be kept small and light.

In another embodiment, the base unit is merged with the imaging system,with the information from the magnetometers delivered through the probescable.

A preferred magnetometric detector comprises several magnetometers,possibly along with an inertial measurement unit, and interfacecircuitry which might be a multiplexer or a microprocessor. Theinterface circuitry enables to pass the signals of multiple probesthrough a single cable or wireless link. It samples the magnetometers(and the inertial measurement unit if present) and possibly monitorsother information such as a state of charge of a battery of themagnetometric detector. By means of a transmitter of the magnetometricdetector, this information is then transmitted to a receiver of the baseunit.

In a preferred embodiment of the invention the magnetometric detectormoreover receives information from the base unit. Thus, preferably,two-way communication is possible between the magnetometric detector andthe base unit. The return channel from the base unit to themagnetometric detector can for example be used to reconfigure themagnetometers or the inertial measurement unit remotely from the baseunit. For example, the working range of the magnetometers can be adaptedto the strength of the magnetic components' magnetic field, inparticular to avoid overflows in the measurement process.

For transmission, the magnetometric detector and the base unit arefunctionally connected with each other. The term “functionallyconnected” encompasses both a direct connection and an indirectconnection through one or more intermediate components, whichintermediate components may be hardware and/or software components.Preferably, the transmission between the magnetometric detector and thebase unit is encoded in a way to prevent eavesdropping, e.g. by means ofasymmetrical encryption. Moreover, preferably measures are taken toprevent interference in the case several systems comprising amagnetometric detector and a base unit are operated in close vicinity.

Preferably, in a calibration step, the magnetometers are calibrated withregard to gain, offset and orientation so that in a homogeneous magneticfield they all yield essentially identical measurements. Thereby it isensured that all magnetometers measure equal values when exposed to ahomogeneous field. For example, a magnetometer rotated in thehomogeneous terrestrial magnetic field should, depending on theorientation of the magnetometer, measure varying strengths of thecomponents of the magnetic field in the three linearly independentdirections. The total strength of the field, however, should remainconstant regardless of the magnetometer's orientation. Yet, inmagnetometers available on the market, gains and offsets differ in eachof the three directions. Moreover, the directions oftentimes are notorthogonal to each other. As described for example in U.S. Pat. No.7,275,008 B2 for a single sensor, if a magnetometer is rotated in ahomogeneous and constant magnetic field, the measurements will yield atilted 3-dimensional ellipsoid. Because the measured field is constant,however, the normalized measurements should lie on a sphere. Preferably,an offset value β and a gain matrix M are introduced to transform theellipsoid into a sphere.

With a set of sensors, additional steps need to be taken to assure thatthe measurements of different sensors are identical with each other. Tocorrect for this, preferably, set of a gain normalisation matrices M_(k)and a normalisation offset vectors β_(k) for each position k aredetermined which transform the magnetometer's raw results a_(k) into anormalized result b_(k):b _(k) =a _(k) *M _(k)+β_(k)

Such a set of gain matrices M_(k) can be obtained by known procedures,for example the iterative calibration scheme described in Dorveaux et.al., “On-the-field Calibration of an Array of Sensors”, 2010 AmericanControl Conference, Baltimore 2010.

By virtue of the defined transformation, b_(k) provides the strength ofthe component of the magnetic field in three orthogonal spatialdirections with equal gain. Moreover, it is ensured that thesedirections are the same for all magnetometers in the magnetometricdetector. As a result, in any homogeneous magnetic field, allmagnetometers yield essentially identical values.

The normalisation information M_(k) and β_(k) for each magnetometer asobtained in the calibration step can be stored either in themagnetometric detector itself or in the base unit. Storing theinformation in the magnetometric detector is preferred as this allowseasy exchange of the magnetometric detector without the need to updatethe information in the base unit. Thus, in a preferred embodiment of theinvention, the magnetometers of the magnetometric device are sampled andtheir results are normalised in the magnetometric detector. Thisinformation, possibly together with other relevant information, istransmitted to the base unit for further analysis.

In another embodiment of the invention, the transformation can beanother, more general non-linear transformation b_(k)=F(a_(k)).

In addition to the above calibration method, another calibration methodis applied which employs an inhomogeneous magnetic field to obtain therelative spatial locations of the magnetometric detector'smagnetometers. While standard calibration methods utilize a homogenousmagnetic field to (a) align the measurement axis of the magnetometersorthogonally, (b) cancel the offset values and (c) adjust to equal gain,it is of further advantage to the described systems that also theprecise relative spatial locations of the magnetometers are available.This can be achieved by an additional calibration step in which themagnetometric detector is subjected to a known inhomogeneous magneticfield. Preferably, comparing the obtained measurements at the variouspositions to the expected field strengths and/or orientations in theassumed locations, and correcting the assumed locations until realmeasurements and expected measurements are in agreement, allows for theexact calibration of the spatial positions of the sensor.

In a variation of the latter calibration method, an unknown rather thana known homogenous field is used. The magnetometers are swept throughthe unknown magnetic field at varying positions, with a fixedorientation. With one of the magnetometers supplying a reference track,the positions of the other magnetometers are adaptively varied in such away that their measurements align with the measurements of the referenceunit. This can be achieved for example by a feedback loop realizing amechano-magnetic-electronical gradient-descent algorithm. The tracksused in this inhomogeneous field calibration can also be composed ofjust a single point in space.

The Imaging Probe and the Processing Unit

Preferably, the magnetometric detector is integral with or removablyattachable to an imaging probe for imaging at least part of the tissueof a patient. “Integral” means that the magnetometric detector ispermanently fixed to the imaging probe in a way that, if the imagingprobe is used as intended, the magnetometric detector cannot be removedfrom the imaging probe. It may, for example, be located inside a housingof the imaging probe. The magnetometric detector may even be joined withthe imaging probe to a degree that for the purpose of service or repairit cannot be separated from the imaging probe without destroying theimaging probe. Yet, the term “integral” also encompasses embodiments inwhich the magnetometric detector can be removed from the imaging probefor the purpose of repair or maintenance. Within the context of thepresent invention, “removably attached” means that one part can beremoved from the other part to which it is attached by a user if thedevice is used as intended. Thus, for example, while the magnetometricdetector remains attached to the imaging probe to be spatiallyassociated to it during a medical procedure, after the medical procedurehas been finished, the detector can be removed from the probe to beattached to another imaging probe for another medical procedure.Preferably, for the purpose of removable attachment, at least one of themagnetometric detector and the imaging probe, preferably both areprovided with one or more fasteners. Preferably, the magnetometricdetector and the imaging probe are attached to each other in a way thatensures that during use in a medical procedure they are in a fixedposition relatively to each other.

The preferred imaging probe is an ultrasound imaging probe. Such anultrasound imaging probe preferably includes an array of ultrasoundtransducers. With the aid of the transducers, ultrasound energy,preferably in the form of ultrasound pulses, is transmitted into aregion of the patient's tissue to be examined. Subsequently, reflectedultrasound energy returning from that region is received and registeredby the same or other transducers. Yet, the present invention may also beused with other types of imaging probes, for example impedance imagingprobes, including probes of the kind disclosed in U.S. Pat. No.7,865,236 B2 to Nervonix Inc.

Other suitable imaging probes include IR sensors or cameras able tomeasure blood flow and/or other scanning devices.

Moreover, preferably, a processing unit is provided. Theposition-related information produced by the magnetometric detector andthe image information produced by the imaging probe preferably istransmitted from the magnetometric detector and the imaging probe,respectively, to the processing unit (the position-related informationpreferably via the base unit as discussed above). The information maythen be combined in the processing unit to generate an image of thetissue of the patient in which image the position of at least a part ofa medical device is indicated based on the position and/or orientationinformation obtained from the magnetometric detector. The preferredprocessing unit comprises a display device for displaying the image. Inthis context, “position-related information” may for example be the rawdata obtained by the magnetometers, the calibrated data or actualpositions and orientations computed as discussed above. Similarly,“image data” may be raw data obtained by the imaging probe or raw datathat has been further processed.

In a preferred embodiment of the invention, the processing unit drivesthe imaging probe and interprets the raw data received from the imagingprobe, e.g. the ultrasound probe. Moreover, preferably, a cable or abundle of cables is provided to connect the imaging probe with theprocessing unit. If the imaging probe is an ultrasound probe, theprocessing unit preferably comprises a driver circuitry to sendprecisely timed electrical signals to the transducers of the imagingprobe in order to generate ultrasound pulses. As part of the ultrasoundpulses is reflected from the region to be examined and returns to theultrasound probe, the received ultrasound energy is converted intoelectrical signals which are then forwarded to the processing unit. Theprocessing unit amplifies and processes the signals to generate an imageof the examined region of the patient's tissue.

As the medical device's positional information is obtained by detectingthe position of the device's magnetic component, the part of the medicaldevice, the position of which is indicated, preferably will either bethe magnetic component or another component of the medical device, theposition of which relatively to the magnetic component is known. In oneembodiment of the invention, only a section of the magnetic componentsis shown, for example the most distal section, such as the distal tip ofa cannula.

For transmission of the information from the magnetometric detector andthe imaging probe (preferably via the base unit) to the processing unit,the former components are functionally connected with the processingunit. Preferably, the processing unit is further functionally connectedto the magnetometric detector (preferably via the base unit) to receiveinformation from the base unit. This way, relevant information may betransmitted from the processing unit or the magnetometric detector tofacilitate the computation that takes place therein.

It can be most useful if the image generating processing unit isconnected in a bi-directional way. Information obtained by processingthe recorded image, preferably in the processing unit, can betransferred to the base unit to facilitate the estimation of the needleposition, and vice versa. For example, a Hough-Transformation on a rawultrasound image might detect a faint image of a needle; thislocalization information from the ultrasound image can be applied asconstraint to the optimization step deriving the same needle positionfrom the magnetometer data. Of course, needle detection algorithmsoperating directly on the image can as well be stabilized through theinformation about needle positions coming from the base unit.

Preferably, in the image it is indicated whether the medical device orpart of the medical device is located inside or outside a pre-determinedspatial plane. Preferably, on the display the image of the patient'stissue in a certain plane is displayed which is identical to thepre-determined spatial plane. The displayed plane may for example bedetermined by the position and/or orientation of the imaging probe. Forexample, if the imaging probe is an ultrasound imaging probe thatoperates in a 2D-mode, the displayed plane is the probe's imaging plane.Preferably, if multiple sections of a medical device are displayed, itis indicated which of the sections is located in the spatial plane,which is located on one side of the spatial plane and which is locatedon the other side of the spatial plane. For example, the imaging planeof the imaging probe can be associated with one colour, one side of theimaging probe with another colour and the other side with a thirdcolour. Alternatively, or in addition, a trajectory of a part of themagnetic component or medical device may be displayed, for example asdisclosed in U.S. Pat. No. 6,733,458 B1. In case the imaging device isprimarily a 3D device, the above description is naturally extended to3D-volumes. Thus, in particular in such case it may be indicated in theimage whether the medical device or part of the medical device islocated inside or outside a pre-determined volume, and on the displaythe image of the patient's tissue in a certain volume may be displayedwhich is identical to the pre-determined spatial volume. The displayedvolume may for example be determined by the position and/or orientationof the imaging probe.

The Magnetic Component and the Medical Device

The magnetic component preferably is integral with the remaining medicaldevice. Alternatively, it may be a replaceable part, for example thecannula of a syringe.

The function of the medical device preferably does not depend on themagnetic component being magnetic. In other words, even if the componentwere not magnetic, the medical device would still perform its purpose.For example, a cannula is not required to be magnetic to serve itspurpose of introducing a fluid into the tissue of the patient. Thisembodiment of the invention exploits the fact that certain components ofmedical devices, that in general are not magnetic, nevertheless have thepotential to be magnetized and can then serve as magnetic components forproviding positional and/or orientation information to a magnetometricdetector.

Alternatively, the magnetic component is a functional component and thefunction depends on the component being magnetic. In this case, theinvention exploits the fact that the magnetic component in addition toserving its magnetism-related function in the medical device can also beused for providing position and/or orientation information.

The inventors have found that magnetic components, in particular such ofmedical devices, can reliably be detected by conventional magnetometersavailable on the market. The magnetic component's magnetic fieldpreferably is not alternating, i.e. it does not periodically change itssign or orientation. The preferred magnetic component's magnetic fieldis not varying in the sense that it keeps its orientation and/orabsolute value with respect to the medical instrument trackedessentially constant during an examination, treatment or surgery. It hasbeen found that such magnetized medical instruments keep theirmagnetization sufficiently constant during a typical medical procedureto be reliably detected by the magnetometric detector.

The preferred magnetic component is at least partly a permanent magnet.Within the context of the present invention, a “permanent magnet” is anobject that is magnetized, thereby creating its own persistent magneticfield due to magnetic remanence. Advantageously, since the magnet ispermanent, no power source is required.

While a permanent magnet is preferred, the invention also encompassesembodiments in which the magnetic component is a non-permanent magnet,e.g. an electromagnet, e.g. a solenoid to which an electric current canbe applied to create the magnetic field. Moreover, in some embodimentsof the invention part of the magnetic component may merely be magneticdue to magnetic induction from another part of the magnetic components,e.g. a permanent magnet-part of the component, while in otherembodiments of the invention such induction does not play a role. Thepart of the magnetic component inducing the magnetic field in the otherpart must not necessarily be integral with the other part. Rather, thetwo parts may be separates. Nor must the two parts necessarily beadjacent to one another, but they can also be at a distance from eachother. In fact, in general, the magnetic component can comprise not onlyone but several separate parts that can be distanced from each other,e.g. several permanent magnets arranged in a row in a medical device.Yet, a component of the medical device that is magnetic merely due toinduction from outside the medical device, e.g. a coil of aradio-frequency antenna, does not create its own magnetic field and istherefore not considered a magnetic component within the context of thepresent invention.

The magnetic components or part of the magnetic components may be amagnetic coating. Preferably, the coating is a permanently magneticcoating. For this purpose, it may for example comprise permanentlymagnetic particles, more preferably nanoparticles. A “nanoparticle” is aparticle that in at least two spatial dimensions is equal to or smallerthan 100 nm in size.

In one embodiment of the invention, the magnetic component has anessentially uniform magnetization. In another embodiment, themagnetization is non-uniform in at least one dimension, i.e. themagnetic moment varies in magnitude and/or direction as a function ofthe location on the magnetic component, thereby creating a one- ormore-dimensional magnetic pattern, e.g. similar to the pattern of aconventional magnetic memory strip (at least one-dimensional) or disk(two-dimensional) as it is used for the storage of information e.g. oncredit cards. In a preferred embodiment of the invention, aone-dimensional magnetic pattern may be recorded along the length of anelongate magnetic component, e.g. a cannula. Advantageously, such apattern can be useful to identify the magnetic component and thus adevice to which it is attached or which it is part of, e.g. the medicaldevice, for documentation purposes. Also, by marking certain parts ofthe medical object with different magnetic codes, these parts can bedistinguished. It is an achievable advantage of this embodiment of theinvention that the position and/or orientation of the magnetic componentcan be better determined, as individual parts of the component can beidentified and individually tracked with respect to their positionand/or orientation. In particular, advantageously, a varying shape ofthe magnetic component, for example a needle bending under pressure, canbe tracked. Moreover, a deformed magnetic component and/or thecomponent's deformation or degree of deformation can be determined moreeasily.

The preferred medical device is elongate, i.e. it is at least twice aslong as it is wide. More preferably, at least the part of the medicaldevice insertable into the patient's tissue is elongate.

Preferably, at least the part of the medical device that is insertableinto the tissue of the patient, more preferably the entire medicaldevice, is tubular. Moreover, preferably the magnetic component ispartially tubular. Alternatively, the part of the medical device that isinsertable into the tissue of the patient or the medical component mayhave a non-tubular shape, for example that of a rod, e.g. if the medicaldevice is an electrosurgical instrument. In a preferred embodiment ofthe invention, at least the part of the medical device that isinsertable, more preferably the entire medical device is a cannula.Moreover, the cannula also constitutes the magnetic component. Thepreferred cannula has a bevelled end with which it is introduced intothe patient's tissue.

The invention can particularly favourably be used with such cannulae, asthey bend easily and therefore the position of the inserted part of thecannula, in particular the cannula tip, cannot easily be determined fromthe position of a needle guide as disclosed in U.S. Pat. No. 6,216,029.As shown in the detailed description below, in a preferred embodiment ofthe invention the model field utilized assumes only two spaced-apartmagnetic charges, one being the needle tip. It has been found that formoderate needle bending, as it normally occurs, the deviations of thereal field from the model field are relatively small; thus the abovemodel can readily be applied for estimating the needle tip's positioneven if the needle is bent.

The invention can particularly advantageously be employed with cannulafor regional anaesthesia. It may, however, also be employed with biopsycannulae and catheters, e.g. catheters for regional anaesthesia. Thepreferred solid material of the cannula or catheter is permanentlymagnetized or the cannula or catheter is provided with a magneticcoating as described above. Alternatively or in addition, the cannula orcatheter may be provided with a solenoid coil, preferably at its distaltip.

The Magnetization Apparatus

The preferred apparatus for magnetizing an elongate medical devicecomprises a magnetization opening. The magnet preferably is located inthe vicinity of the magnetization opening to magnetize the elongatemedical device as it passes through the opening. Preferably the openingin the apparatus is an opening in the reservoir, through which openingthe elongate medical device can be withdrawn from the reservoir so thatwhen the elongate medical device is removed from the reservoir it ismagnetized. Preferably, the elongate medical devices are kept in asterile packaging different from the reservoir while they are stored inthe reservoir. Preferably, they remain in this packaging while they aremagnetized.

The preferred elongate medical device is a cannula a rod or a needle.The preferred reservoir can hold more than one elongate medical devicesimultaneously.

The magnet preferably is an electromagnet, e.g. a solenoid, morepreferably a ring-shaped solenoid electromagnet. Alternatively, it maybe a permanent magnet. The magnetizing device can be designed touniformly magnetize the inserted medical device, or to record amagnetized pattern along the elongated medical object. This, forexample, may be useful to identify the medical object used fordocumentation purposes as discussed above. Preferably, a variation ofthe magnetisation along the length of the elongate medical device can beachieved by varying the solenoid's magnetic field as the elongatemedical device progresses through the magnetization opening. To controlthe variation of the magnetic field, the progress of the elongatemedical device can for example be recorded by a measurement roll that isin contact with the elongate medical device when it passes through themagnetization opening. In another embodiment, the magnetization deviceis a hollow tube made up of separate segments of the magnet. By applyingdifferent currents to different sections of the tube, a magnetic patterncan be inscribed on the medical instrument.

The apparatus may be further developed into a calibration tool byproviding markings on the apparatus for alignment of the magnetometricdetector and the elongate medical device. Preferably, for alignment themagnetometric detector is integral with or attached to the imaging probeand the imaging probe is aligned with the elongate medical device usingthe markings. Thus, the magnetometric detector is aligned with themedical device via the imaging probe.

By virtue of the markings, the elongate medical device and themagnetometric detector can be put in a well-defined relative position.From this, based on the measurements of the magnetometric devicespecific parameters of the elongate medical device, most preferably itslength and magnetic momentum can be measured for facilitating the latercomputation of the position and orientation of the cannula during themedical procedure. In fact, these parameters can greatly enhance fittinga model to the parameters measured by the magnetometers as describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in greater detail with the aid of schematicdrawings:

FIG. 1 shows schematically an imaging system comprising an imagingprobe, a magnetometric detector and a medical device according to theinvention;

FIG. 2 shows a block diagram of a magnetometric detector according tothe invention;

FIG. 3 shows a block diagram of a base unit according to the invention;

FIG. 4 shows schematically three examples of images of the patient'stissue with the position and orientation of cannula superimposedaccording to a first embodiment of the invention;

FIG. 5 shows schematically a magnetometric detector according to asecond embodiment of the invention;

FIG. 6 shows a system of an ultrasound imaging probe and themagnetometric detector according FIG. 5;

FIG. 7 shows the absolute gradient field strength in the embodiment ofFIGS. 5 and 6 as a function of the needle's distance from the imagingplane of the ultrasound imaging plane; and

FIG. 8 shows a magnetization apparatus for magnetizing a cannulaaccording to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The imaging system 1 shown in FIG. 1 comprises a hand held ultrasoundprobe 2 as an imaging probe connected via a cable 3 with a processingunit 4. The processing unit 4 drives the ultrasound probe 2, i.e. itsends electrical signals to the ultrasound probe 2 to generateultrasound pulses and interprets the raw data received from theultrasound probe 2 to assemble it into an image of the patients tissuescanned with the ultrasound probe 2. Moreover a battery operatedmagnetometric detector 5 is, by means of a Velcro fastener (not shown)attached to the ultrasound probe 2. Positioning elements are provided onthe magnetometric detector 5 to ensure that whenever it is attached anewto the ultrasound probe 2 it is always attached in the same well-definedposition and orientation.

The magnetometric detector 5 comprises magnetometers 14, 15 (not shownin FIG. 1) and is wirelessly or by other means connected with a baseunit 6 in a bi-directional manner (indicated by the flash symbol 7). Forthis, both the magnetometric detector 2 and the base 6 unit are providedwith wireless transceivers. The transceivers may for example employ theBluetooth™ standard or a standard from the WiFi (IEEE 802.11) family ofstandards. The base unit 6 receives the normalized results of themeasurements of the magnetometric detector 2 and from this calculatesthe position or, in some embodiments the position and orientation of amagnetic medical cannula 8. Along with the measurement results,additional information such as the state of charge of the magnetometricdetector's 5 battery is transmitted from the magnetometric detector 5 tothe base unit 6. Moreover, configuration information is transmitted fromthe base unit 6 to the magnetometric detector 5.

The base unit 6 forwards the result of its calculation, i.e. theposition or in some embodiments the position and orientationinformation, to the processing unit 4. For this purpose, the base unit 6may for example be connected with the processing unit 4 via astandardized serial connector such as a USB™ (Universal Serial Bus)connector, a FireWire™ (also referred to as iLink™ or IEEE1394)connector or a Thunderbolt™ (also referred to as Light Peak™) connector.In the processing unit 4, the information received from the base unit 6and the ultrasound image are combined to generate on a display screen 9of the processing unit 4 an image of the tissue of the patient in whichthe current position of the cannula 8 in the tissue is indicated.Moreover, the base unit 6 receives configuration information and/or aprior information about the position of the cannula 8 from theprocessing unit 4 through the same connection.

The components of the magnetometric detector 5 are shown schematicallyin greater detail in the block diagram of FIG. 2. The magnetometricdetector 5 comprises an array 10 of two or more (e.g. four)magnetometers 14, 15 (not shown in FIG. 2) that is sampled by amicroprocessor 11. The microprocessor 11 normalizes the measurementresults obtained from the magnetometer array 10 and forwards it to atransceiver 12 with an antenna 13 which, in turn transmits theinformation to the base unit 6. In a modified version of thisembodiment, the magnetometric detector 5 is provided with a multiplexerrather than with a microprocessor 11 and the normalization is performedby a processor 18 in the base unit 6.

Each magnetometer 14, 15 in the array 10 of magnetometers 14, 15measures the components a_(k) ^(u), a_(k) ^(v), a_(k) ^(w) (k indicatingthe respective magnetometer) of the magnetic field at the position ofthe respective magnetometer 14, 15 in three linearly independentdirections. The microprocessor 11 transforms these raw valuesa _(k)=(a _(k) ^(u) ,a _(k) ^(v) ,a _(k) ^(w))into corresponding normalized valuesb _(k)=(b _(k) ^(x) ,b _(k) ^(y) ,b _(k) ^(z))in predetermined orthogonal directions of equal gain by multiplying thethree values a_(k) obtained from the magnetometer with a normalisationmatrix M_(k) and adding a normalisation offset vector β_(k):b _(k) =a _(k) *M _(k)+β_(k)

This same transformation is performed for all magnetometers with theirrespective normalisation matrix and adding a normalisation offset vectorsuch that the result b_(k) for each magnetometer provides the componentsof the magnetic field in the same orthogonal spatial directions withidentical gain. Thus, in a homogenous magnetic field, all magnetometersalways provide identical values after normalisation regardless of thestrength or orientation of the homogenous magnetic field. Thenormalisation matrices and the normalisation offset vectors arepermanently stored in a memory associated with the microcontroller.

The base station 6 shown schematically in greater detail in FIG. 3receives the normalised positional information from the magnetometricdetector 5 through its receiver 16 with antenna 17 and forwards theinformation to a processor 18. There, the normalized results of themeasurements are combined to derive the position (or position andorientation) of the cannula 8. For this purpose, the values b_(k) arefit to a model of the combined magnetic field originating from themagnetic cannula 8 and the terrestrial magnetic field. The unknownparameters p in this model are the cannula's location 1 relatively tothe ultrasound probe, it's length and orientation d and it's magneticcoercivity m as well as the terrestrial magnetic field E:p={l,d,m,E}.

The unknown parameters are obtained by means of the model of themagnetic field of the magnetic cannula and the terrestrial magneticfield, whereinc _(k)(p)=(c _(k) ^(x)(p),c _(k) ^(y)(p),c _(k) ^(z)(p))are the normalized components of the magnetic field according to themodel at the position of magnetometer k at a given set of parameters p.By means of appropriate algorithms known to the skilled person theparameters p are obtained at which the deviation of the components ofthe magnetic field according to the model from the components actuallymeasuredΣ_(k)(b _(k) −c _(k)(p))²is minimized. Suitable minimization techniques are for examplegradient-descent algorithms as well as Levenberg-Marquardt approaches.Moreover, Kalman filter techniques or similar iterative means can beutilized to continuously perform such an optimization.

If the cannula is sufficiently rigid, i.e. it does bend only slightly,it can be approximated as a straight hollow cylinder. The magnetic fieldof such cylinder is equivalent to that of opposite magnetic charges(i.e. displaying opposite magnetic force) evenly distributed on the endsurfaces of the cylinder, i.e. two circular rings at the opposite endsof the cannula, the rings having opposite magnetic charge. In view ofthe small diameter of the cannula, the charges can further beapproximated by two magnetic point charges at the opposite ends of thecannula. Thus, according to the model, the magnetic field of a cannulaextending along the vector d is measured from a position r_(k) isN(r _(k) ,d,m)=m*(r _(k) /|r _(k)|³−(r _(k) +d)/|r _(k) +d| ³).

Here |r_(k)| and (|r_(k)+d| indicate the absolute values of the vectorsr_(k) and r_(k)+d, respectively. The positions r_(k) can be converted tothe location 1 of the cannula 8 relatively to the ultrasound probe 2with the help of the known positions of the magnetometers 14, 15 in themagnetometric detector 5 and the position of the magnetometric detector5 relatively to the ultrasound probe 2. Consequently, furtherconsidering the terrestrial magnetic field E, the components of themagnetic field according to the model amount toc _(k)(p)=N(r _(k) ,d,m)+E=m*(r _(k) /|r _(k)|³−(r _(k) +d)/|r _(k) +d|³)+E

Note that in contrast to many known approaches the above model does notassume the field of the needle to be a dipole field. This would be anoversimplification as the magnetometric detectors in general are tooclose to the needle as compared to the length of the needle to make adipole field a valid approximation.

The values obtained by fitting the model to the actual values detectedby the magnetometers 14, 15 as described above are then forwarded viadata interface 19, e.g. a USB™ connector, to the processing unit 4.There, they are superimposed on the image of the tissue as obtained fromthe handheld ultrasound probe 2. The method of how the cannula 8 isvisualized on the display screen is discussed with reference to FIG. 4.FIG. 4b shows the cross section of a blood vessel 20 as imaged by thehand held ultrasonic probe 2 in 2D mode. Accordingly, the blood vessel20 cuts through the imaging plane of the ultrasound probe 2. Moreover,schematically, it is shown how the cannula 8 is visualized depending onits position relatively to the imaging plane. The cannula is alwaysvisualized as a line, the end of which corresponds to the cannula's tip.If the cannula 8 is within the probe's 2 imaging plane, it is, of afirst colour (indicated as a full line 21 in the figures). If, on theother hand, the cannula 8 is outside the imaging plane, it isnevertheless shown, albeit in a different colour, the colour dependingon whether the cannula 8 is in front of (colour indicated as a dashedline 22 in the figures) or behind the imaging plane (colour indicated asa dotted line 23 in the figures). FIG. 4a shows the situation when thecannula 8 cuts through the imaging plane. In this case, the section ofthe cannula 8 behind the imaging plane is shown in a different colourthan the part of the cannula 8 that cuts through the plane which againhas a different colour to the part of the cannula 8 that is in front ofthe imaging plane. The situation in FIG. 4c differs from that in FIG. 4aonly in that the cannula 8 cuts through the plane in a different angle.The sections of the cannula 8 outside the imaging plane are shown on thedisplay as their projections vertically onto the imaging plane.

In another embodiment, the whole expected needle track is shown on theimage display, as described above. The actual position of the needle isindicated either by a different colour or line style (bold/hatched/etc)of the needle track. Furthermore, the point of cutting through theimaging plane might be indicated by a special graph for example, eitherthe circle shown in FIG. 4a or a rectangle. The form or appearance ofthe graph might change to indicate the probability of the needlepiercing the plane at that point, i.e. instead a circle a generalellipse might be used to indicate the target area.

An alternative embodiment of the magnetometric detector is shown inFIGS. 5 and 6. This magnetometric detector in a first variant of theembodiment only comprises a set 5 two magnetometers 14, 15. In analternative variant of the embodiment one or more further sets areprovided in further locations on the ultrasound probe 2. It is possibleto derive whether the cannula 8 is located within the imaging plane 24,in front of 25 the imaging plane or behind 26 the imaging plane 24. Forthis purpose, the magnetometers are arranged along a line parallel tothe probe's longitudinal axis. The normalized measurement results of thefirst magnetometer 14 are subtracted from those of the secondmagnetometer 15, thereby effectively cancelling out the terrestrialmagnetic field. The difference is essentially pointing into thedirection of the needle tip, because the other field component, causedby the end of the needle, is rapidly decaying in distance from thesensor arrangement. Thus, the sensor essentially “sees” only the needletip. A relative distance can be inferred through the magnitude of themeasured difference field.

In another embodiment of the invention, the magnetometers 14,15 arearranged perpendicular to the probe's longitudinal axis. Thus, thedifference obtained essentially is the gradient of the magnetic fieldgenerated by the magnetic cannula 8. By analyzing the magnitude of thegradient, a relative distance of the cannula from the sensor can beelucidated. By analyzing the direction of the gradient it can beelucidated if the cannula 8 is in front of 25 or behind 26 or directlyon the imaging plane 24.

FIG. 7 shows the absolute gradient field strength G (in arbitrary units)of a cannula 8 that extends in parallel to the imaging plane 24 but at adistance Y (in arbitrary units) from this plane 24. As can be seen, thegradient field strength G has a minimum if the distance Y equals 0, thatis if the cannula 8 is in the imaging plane 24. If, on the other hand,the gradient field strength G is above a certain threshold, the cannula8 can be assumed to be outside the imaging plane 24. In this case, thedirection of the gradient field indicates whether the cannula 8 is infront of 25 or behind 26 the imaging plane 24 (this is not shown in FIG.7 as the figure only shows the absolute value of the field). Thus, thissimple setup can be used to for example superimpose on the ultrasoundimage displayed on the screen of the processing device the cues “*” (inplane), “=>” (in front of the imaging plane) or “<=” (before the imagingplane) even though the exact location and position of the needle ofcourse cannot be indicated.

Finally, FIG. 8 shows an apparatus 27 for magnetizing cannulae 8according to the invention. Inside the box-shaped apparatus is areservoir (not shown) which can hold a number of cannulae 8, eachcannula 8 enclosed in a separate sterile film packaging 28. Theapparatus 27 moreover comprises a round opening 29 through whichindividual cannulae 8 with their film packaging 28 can removed from thereservoir. On the inside, the opening is surrounded by a ring-shapedsolenoid electromagnet (not shown). The electromagnet is powered by apower supply (not shown) attached to the apparatus. A switch on thepower supply can electrify the electromagnet and thus turn on anelectromagnetic field. Then, if a cannula 8 is removed from the boxthrough the opening, it is at the same time magnetized.

Appropriate modulation of the electromagnet's current will allow codedmagnetization as the needle is with-drawn from the reservoir.

Alternatively, the opening is one side of a hollow cylinder composed ofseparate magnetizing coils which allow imprinting a magnetic code ontothe medical device in one step.

Subsequently, in order to measure the cannula's 8 magnetic moment andlength, it is, still enclosed in the transparent sterile film packaging28, placed on a line-shaped marking 30 on the apparatus 27. Theultrasound probe 2 within the attached magnetometric detector 5 isplaced on another, box-shaped, marking 31 on the apparatus 27. As, fromthis, the relative position and orientation of the magnetometric device5 and the cannula 8 is known, the magnetic moment and the length of thecannula can easily be derived from the measurements of the magnetometers14, 15 after normalization. These values can then be used during themedical procedure to facilitate deriving the position and orientation ofthe cannula 8 from the measurements of the magnetometers by means of theabove-described model.

The features described in the above description, claims and figures canbe relevant to the invention in any combination.

The invention claimed is:
 1. A method of obtaining at least one ofposition information and orientation information about a cannula of amedical device, the method comprising: magnetically coding the cannula,the cannula formed at least in part of a material that can bemagnetized, by exposing the cannula to a magnetic field to inscribe intothe cannula a coded magnetization pattern along the length of thecannula, to cause the cannula to radiate the magnetically-codedidentification pattern; detecting with a magnetometric detector at leastone of position information and orientation information of the cannula;and identifying the cannula by the magnetically-coded identificationpattern.
 2. The method according to claim 1, wherein the magnetometricdetector is integral with an imaging probe for imaging patient tissue.3. The method according to claim 1, wherein the magnetometric detectoris removably attachable to an imaging probe for imaging patient tissueand the method further includes attaching the magnetometric detector tothe imaging probe.
 4. The method according to claim 1 wherein the stepof magnetically coding the cannula of the medical device by exposing thecannula to a magnetic field comprises locating the cannula in thevicinity of a magnet.
 5. The method according to claim 1 wherein thestep of magnetically coding the cannula of the medical device byexposing the cannula to a magnetic field comprises locating the cannulain a varying magnetic field to create the coded magnetization pattern onthe cannula.
 6. The method according to claim 4 wherein the step ofmagnetically coding the cannula of the medical device by exposing thecannula to a magnetic field comprises locating the cannula on amagnetizer comprising a magnet to create the coded magnetization patternon the cannula.
 7. The method according to claim 6 wherein the codedmagnetization pattern is a one-dimensional magnetic pattern.
 8. Themethod according to claim 6 wherein the coded magnetization pattern is amore-than-one-dimensional magnetic pattern.
 9. The method according toclaim 6 wherein the magnet is one of: a permanent magnet or a solenoidelectromagnet, the magnet being adapted to record the codedmagnetization pattern along the cannula.